1. Field of the Invention
This invention relates to coherent pulse doppler synthetic aperture radars, and more particularly to an improved radar aircraft speed determining system.
2. Description of the Prior Art
Coherent pulse doppler synthetic aperture radars of the radar ground mapping type are well known in the art. As is known, the principle object of one form of such a radar is to derive high resolution maps over a large area of terrain by means of processing of the coherent return receive from an illuminator (a radar transmitting antenna) the illuminated area of which is much greater than the desired map resolution. In such processing radars, the returns received by the radar antenna are separated in accordance with range, thereby providing a plurality of range strips which are at distinct radial distances from the illuminator, or radar. Such systems also separate signals in accordance with the doppler frequency of return signals received, which frequency is the difference between the frequency of the transmitted wave and the frequency of the received wave which results from compression or expansion of the wave as a result, in turn, of a closing or opening velocity of the antenna with respect to a given reflector. Naturally, a map is defined when each resolvable point of intensity is expressible by the analysis of signals having in the information content thereof, designations of range and a second coordinate such as relative angle with respect to the antenna.
In the case of forward-looking, or squint radar, the doppler frequency of a given target changes instant by instant as a result of the fact that the relative bearing of the target to the aircraft (the angle in azimuth) changes instant by instant as a result of the aircraft's velocity. Thus, the doppler frequency changes quite rapidly, and more rapidly for targets at short range than for targets at extremely long range, on a relative basis. In order to accurately filter the return signals so as to achieve high resolution in doppler, it is desirable to utilize a long sampling interval and a doppler filter having a long time constant. For finite targets close to the radar, the target moves from one resolvable doppler frequency to another very rapidly. In order to achieve a long sampling interval at a given doppler frequency, it is therefore necessary to alter the data to make it appear as if it is frozen on the surface of a given doppler frequency cone throughout the sampling interval. This is achieved by what is referred to as focusing, including a parabolic focus term which continuously adds a phase angle to a given return signal so as to make it appear to be relatively static with respect to the aircraft (as might a distant start) throughout the processing interval. As the aircraft goes faster, more doppler focusing is required; as the aircraft goes slower, less focusing is required. The focusing applied to the return signals must therefore be governed as a function of aircraft speed.
In addition to focusing, resolution of the map (which is given in terms of doppler cone angle and range) into an orthogonal coordinate system, such as referred to North and East on the earth's surface, requires a very accurate measure of speed; for instance, in certan applications today, the degree of accuracy in map making may require a measurement of speed accurate to one part in several thousand. This speed measurement must be available during the time that the data is acquired for the map in order to resolve the map as a function of the velocity vector of the aircraft as the data is being collected through the radar system. In other words, this is not a factor which can be corrected later or which may be corrected in ground-based computer systems. It is also obvious that, if mapping information is to be utilized for navigational or weapon delivery purposes, the map must be accurately made on a real time basis while the aircraft is in flight, and therefore accurate speed determination must similarly be made in a real time basis during the mapping procedure. Prior art systems utilize the speed indications provided by the navigational system of the aircraft. However, since the speed can be determined only to a degree of accuracy which is less than the accuracy that may be required for a high resolution map, the focusing of return signals and orthogonal coordination may be sufficiently inaccurate so as to create intolerable errors in map generation.
Similarly, more focus correction is needed for targets which are close since the doppler frequency cone angle changes very rapidly, whereas very distant targets require less increase in focus or less correction. Therefore, focus is applied differently to each range gate.
As is derived in a copending application of the same assignee entitled ACCURATE SPEED AND CONE ANGLE MEASUREMENT IN A COHERENT PULSE DOPPLER SYNTHETIC APERTURE RADAR SYSTEM, Ser. No. 87,011, filed on Nov. 4, 1970 by Donald Richman, the relationship between speed (S), range (R), range rate (R) and range acceleration (R) is EQU S.sup.2 =RR+R.sup.2 ( 1)
the R term relates to doppler cone angle, by EQU R=.lambda./2f.sub.d, (1a)
where f.sub.d= the doppler frequency and is quite accurately determined. The range (R) can be accurately determined from range gating. It is the range acceleration (R) which relates to focus, and which is only approximately accurate.